What does an assembly line assemble?

If you have to give a demonstration in class, discuss the filling of bottles in a food factory. The bottles are washed and dried in one machine and them move along a conveyor to the filling machine which fills that with the milk or Coca Cola. Once filled, the bottles move to the capping machine where the cap is fitted. Once capped, the bottles go along the conveyor to the packing station where they have their "Best By Date" stamped on them before being packed in the boxes or crates.

As the other person said, almost anything can and is "put together" on an assembly line. This includes cars and television sets.

A cafeteria is a type of assembly line. You are assembling your lunch. You could demonstrate an assembly line by the progressive construction of a row of Double Whoppers with cheese by having several people each provide one of the ingedrients.

Just about every kind of mass produced product.